Etydhv

There is no plausible way of writing a complete and accurate history of events after Robert's rebellion without mentioning Tyrion. It was likely a joke.

The man has been the Hand of the King to Joffrey I Baratheon and Brandon I Stark. He has been convicted of killing a King and his own father who happened to be the Hand of the King and the most powerful man in Westeros.

His association with Dany would earn him a long chapter dedicated to him as well.

The history books of Westeros, like those of the corresponding era of Europe, do not prize accuracy above all else. Therefore, unless Maester Ebrose was unusually scrupulous, he would be focusing not only on which events actually occurred, but also on telling a story. He would want this story to have a satisfying and tidy narrative, so that people would read and enjoy his book, and he would want to tell a story that was acceptable in the political climate he was writing in, so that he could keep his enviviable position as archmaester.

History books focus on figureheads

People like reading stories about Lords and Ladies, Princes and Princesses, Kings and Queens. Their entourages are of less interest. No matter how important they are behind the scenes, advisors and councillors are unlikely to get a mention. Even for a position as important as Hand of The King, it makes for a better story to simply pretend that it was the monarch themselves who made the decisions and gave the commands. This is highlighted rather touchingly by Varys after the Battle of Blackwater.

There are many who know that without you this city faced certain defeat. The King won't give you any honours, the histories won't mention you, but we will not forget.

Varys' prediction was correct. Although Tyrion was highborn and Hand to several Kings and Queens, he was never the ruler of any of the great houses or any of the Seven Kingdoms. (This situation might have changed since the end of Ebrose's book, due to the deaths of every other member of House Lannister.) Therefore his actions would not necessarily merit inclusion in a history.

Tyrion's victory at Blackwater is King Joffery's victory. The fact that Tyrion was convicted of Joffery's murder is irrelevant since it was later revealed that the murderer was Lady Olenna Tyrell. The commands Tyrion gave as Hand of The Queen would have been attributed to Daenerys Targaryen. The one action that he might have been granted credit for was the murder of his father Tywin, but (assuming it was even public knowledge) Maester Ebrose would have found it awkward to include this fact.

It would have been politically unwise to mention him

Westeros is now totally controlled by the Starks. They rule the Night's Watch, The North and The Six Kingdoms. Furthermore they've appointed a close personal friend as Grand Maester. Therefore Maester Ebrose would wish to avoid publishing anything that would offend them or cast them in a bad light.

Likewise Tyrion is in a very powerful position as Hand of The King, so Maester Ebrose would also wish to avoid offending him.

This puts Ebrose in a difficult position since Tyrion spent almost all of the wars serving those fighting against the Starks. If he mentioned Tyrion in a flattering way it would make the Starks look worse. If he mentioned Tyrion in an unflattering way then Tyrion might have retaliated. It's hard to mention him in a neutral way because Tyrion spent all his time associating with the Starks' enemies. Worst of all, Tyrion's main achievement was his victory at Blackwater against Stannis, the very man that Ned Stark was executed for supporting.

So Maester Ebrose took the easy solution and simply didn't mention Tyrion at all. This ran the slight risk of offending Tyrion by diminishing his importance, but the risk was less than that of writing something that could have directly offended Tyrion or the Starks.

I see people are trying to come up with in-universe explanations, but I strongly think that the reason is this:

The writers simply wanted to make yet another joke within the larger joke that was the whole scene.

Besides amusement, they also wanted to emphasise that Tyrion played a huge role in obtaining the final peace, and ironically nobody would ever know.

This would be a nod to how the Maesters are not men of science, as we now know them, nor even historians as we now know them, as befits such men in the time period and setting most similar to their setting from our own history. The story has been recorded not to provide an accurate and factual retelling, but to provide the story. It was, apparently, this Maester's decision that Tyrion did not fit into the story he wanted to tell. A whore-mongering imp was not quite as stirring a character as warranted inclusion, perhaps.

The book was on the war of succession, and Tyrion did not play a big role in that. He did not try to claim the throne, he was not politically important as he was not a head of a house, and he did not lead armies in any significant battles.

His act as a hand will be attributed to the rulers he served, his marriage to Sansa was inconsequential, and the murder of Joffery and Tywin were probably attributed to their enemies.

Him being named hand of Brandon is probably outside of the scope of the book.

I believe this book is to mimic the books we read (also titled (as a whole) A Song of Ice and Fire). In the books Tyrion is certainly mentioned- but they make certain to spend a good deal of rime describing how hideous looking Tyrion is. I took this joke in the show to be Sam saying “no, no, you’re not even in it..” all sheepish and then Tyrion reading parts, smirking, rolling his eyes and closing the book- to be him seeing his description as a horrible looking moster-esque man and that was the joke...